programming-examples/java/Data_Structures/Java Program to Implement Rolling Hash.java

/**
 * Java Program to Implement Rolling Hash
 **/

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Random;
import java.math.BigInteger;

class RabinKarp
{
    /** String Pattern **/
    private String pat;
    /** pattern hash value **/
    private long patHash;
    /** pattern length **/
    private int M;
    /** Large prime **/
    private long Q;
    /** radix **/
    private int R;
    /** R^(M-1) % Q **/
    private long RM;

    /** Constructor **/
    public RabinKarp(String txt, String pat)
    {
        this.pat = pat;
        R = 256;
        M = pat.length();
        Q = longRandomPrime();
        /** precompute R^(M-1) % Q for use in removing leading digit **/
        RM = 1;
        for (int i = 1; i <= M-1; i++)
            RM = (R * RM) % Q;
        patHash = hash(pat, M);
        int pos = search(txt);
        if (pos == txt.length())
            System.out.println("\nNo Match\n");
        else
            System.out.println("Pattern found at : "+ pos);
    }
    /** Compute hash **/
    private long hash(String key, int M)
    {
        long h = 0;
        for (int j = 0; j < M; j++)
            h = (R * h + key.charAt(j)) % Q;
        return h;
    }
    /** Funtion check **/
    private boolean check(String txt, int i)
    {
        for (int j = 0; j < M; j++)
            if (pat.charAt(j) != txt.charAt(i + j))
                return false;
        return true;
    }
    /** Funtion to check for exact match**/
    private int search(String txt)
    {
        int N = txt.length();
        if (N < M) return N;
        long txtHash = hash(txt, M);
        /** check for match at start **/
        if ((patHash == txtHash) && check(txt, 0))
            return 0;
        /** check for hash match. if hash match then check for exact match**/
        for (int i = M; i < N; i++)
            {
                // Remove leading digit, add trailing digit, check for match.
                txtHash = (txtHash + Q - RM*txt.charAt(i - M) % Q) % Q;
                txtHash = (txtHash * R + txt.charAt(i)) % Q;
                // match
                int offset = i - M + 1;
                if ((patHash == txtHash) && check(txt, offset))
                    return offset;
            }
        /** no match **/
        return N;
    }
    /** generate a random 31 bit prime **/
    private static long longRandomPrime()
    {
        BigInteger prime = BigInteger.probablePrime(31, new Random());
        return prime.longValue();
    }
}

/** Class RollingHashTest **/
public class RollingHashTest
{
    public static void main(String[] args) throws IOException
    {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Rolling Hash Test\n");
        System.out.println("\nEnter Text\n");
        String text = br.readLine();
        System.out.println("\nEnter Pattern\n");
        String pattern = br.readLine();
        System.out.println("\nResults : \n");
        RabinKarp rk = new RabinKarp(text, pattern);
    }
}

/*
Enter Text

abcdefghijklmnopqrstuvwxyz

Enter Pattern

jklmn

Results :

Pattern found at : 9